Why do you think it is fan noise? It just might be the fuel pump.
The transceivers you state are both FM units. Therefore, they shouldn't be able to hear the hash if it were the fans. In fact, if it were the fuel pump, the receiving frequency is so high that I'd doubt you'd hear it.
What I suspect is, a poor ground causing an intermittent ground loop allowing what little alternator ripple or regulator harmonics to creep in.
This said, without some more detail with respect to how the radios are mounted and wired; what antennas are being used and how they are mounted; and perhaps some detail of what the noise sounds like, diagnosing the problem is a little difficult.
I have a few examples of fan and fuel pump noise on my web site if you're interested. Look under noise ID.